The present invention relates to a process and novel catalyst for the production of aromatics from a feed stream comprising C2-C6 aliphatic hydrocarbons. The disclosed dehydrocyclodimerization process uses a zeolite catalyst having an essentially inert binder (e.g. silica). Minimization of non-selective acid sites of the binder material significantly improves both the catalyst activity and stability.
Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce an aromatic product and hydrogen. Normal downstream separations will typically yield, in addition to the major C6-C9 aromatic product stream, a light ends byproduct containing hydrogen for purge and recycle, an unconverted C2-C4 product for recycle, and a trace C4+ non-aromatic byproduct. This process is well known and the associated background and details are given in U.S. Pat. No. 4,654,455 and U.S. Pat. No. 4,746,763, hereby incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500xc2x0 C. using dual functional catalysts containing acidic and dehydrogenation components.
In British patent 1496379, the acidic function is provided by a surface active oxide such as hydrated silica or hydrated alumina having hydroxyl groups that may be ion-exchanged with a metal having the requisite dehydrogenation function (e.g. gallium). Alternatively, the metal may be impregnated onto the acidic support as a metal oxide. In this case, ion exchange capacity of the support is not needed, so that various forms of alumina (e.g. eta-alumina) may be used. Silica is also mentioned as a suitable support, although this material alone lacks the appreciable acidity required for the dehydrocyclodimerization reaction. Trace amounts of contaminant alumina may improve the support acidity, but not significantly. In U.S. Pat. No. 4,157,356, silica is again referenced as a preferred catalyst support, either with or without surface hydroxyl groups, depending upon whether metal loading is to be achieved via ion-exchange (former case) or metal oxide impregnation (latter case). The silica support here is characterized as having a surface area of greater than 500 m2/g and a pore volume of less than 0.8 ml/g.
In U.S. Pat. No. 4,855,522, a zeolitic catalyst composition for the production of aromatics from C2, C3, and C4 paraffinic hydrocarbons is disclosed. Specifically, the catalyst comprises a crystalline aluminosilicate having a molar SiO2/Al2O3 ratio of at least 5:1 and is loaded with both a gallium compound and at least one rare earth metal (e.g. lanthanum). The novelty of this formulation lies in the rare earth additive, which is taught to improve aromatic selectivity compared to that achieved using conventional gallium zeolitic catalysts. It is further mentioned that the zeolite may be bound with silica or alumina. A second example of a zeolite and gallium containing catalyst that may be bound with silica is provided in U.S. Pat. No. 4,350,835 for converting gaseous feed stocks containing ethane to liquid aromatics.
For the reaction of concern in the present invention, however, namely the dehydrocyclodimerization of C2-C6 aliphatic hydrocarbons, the prior art fails to disclose and recognize the advantages associated with a gallium containing zeolite catalyst having an essentially non-acidic binder. Appropriate binders are selected from the group consisting of silica, zirconia, titania, and mixtures thereof. Because these materials are essentially devoid of acidic sites, undesired side reactions leading to reaction products other than C6-C9 aromatic compounds are greatly reduced. Such non-selective products include cracked hydrocarbons such as methane as well as high boiling aromatic compounds such as naphthalene. Furthermore, the acidity of the zeolite component of the catalyst may be reduced during synthesis through a steaming procedure to cause a desired amount of zeolite dealumination. Overall, the substantial improvement in selectivity to aromatics obtained in the dehydrocyclodimerization process of the present invention has important commercial implications in terms of product yields and catalyst life.
Of significant industrial importance currently for this dehydrocyclodimerization process is the catalyst disclosed in U.S. Pat. No. 4,636,483 comprising a crystalline aluminosilicate, gallium, and phosphorus containing alumina as a binder. Compared to other known dehydrocyclodimerization catalysts, a principal benefit of this composition is the binder material, which increases aromatic selectivity and also significantly improves catalyst life through a reduction in detrimental carbonaceous byproduct (i.e. coke) formation. Furthermore, the aluminum phosphate binder is conveniently prepared by combining an alumina hydrosol with a phosphorous compound to modify the sol prior to gellation. Details of this procedure are provided in U.S. Pat. No. 4,629,717.
One particular drawback associated with the use of this phosphorous containing alumina binder, however, is described in U.S. Pat. No. 5,212,127 where catalysts incorporating this material are subject to deactivation through extended exposure to hydrogen at temperatures exceeding 500xc2x0 C. Unfortunately, the preferred environment for the dehydrocyclodimerization reaction encompasses these conditions. Applicants have found that the use of an essentially non-acidic binder (e.g. silica) rather than phosphorous containing alumina in the catalyst composition described in U.S. Pat. No. 4,636,483 prevents the aforementioned deactivation due to high temperature hydrogen exposure. Thus, the need for either a catalyst reactivation step as described in the ""127 patent or a pretreatment as described in U.S. Pat. No. 5,169,812 is eliminated.
The prior art catalyst formulation using an aluminum phosphate binder provides some degree of selectivity improvement over catalysts incorporating a more acidic binder, such as a conventional alumina binder. While aluminum phosphate is less acidic than alumina, it nevertheless contains some acidity that is detrimental in the dehydrocyclodimerization process of the present invention. It is now understood that an essentially complete elimination of catalyst binder acidity is necessary to minimize unwanted side reactions. In accordance with this understanding, the use of a nonacidic binder for the zeolitic catalyst of the present invention represents a further advancement in the art regarding dehydrocyclodimerization catalyst coking reduction. Applicants have found that the essential elimination of acidic sites on the binder surface greatly reduces the production of acid-catalyzed, non-selective heavy aromatics (e.g. naphthalenes) that are known coke precursors. Confining catalyst acidity, which is necessary to carry out the dehydrocyclodimerization reaction, to the zeolite micropores yields relatively more of only the desired shape selective aromatic products (e.g. benzene, toluene, xylenes) and relatively fewer of the non-selective coke precursors. Larger molecules do not easily diffuse from the microporous channels of the medium pore (5-6 xc3x85) zeolite used in the catalyst of the present invention. During catalyst preparation, steaming of the zeolite can be effective for reducing acidity of the zeolite itself through dealumination.
Overall, compared to prior art formulations, the gallium containing zeolite catalyst having an essentially non-acidic binder provides a significantly reduced deactivation rate under dehydrocyclodimerization conditions, which is a principal object of this invention. This stability enhancement results from both a decreased production of coke-forming byproducts as well as high tolerance to the deleterious effects of high temperature hydrogen exposure. As a result, regeneration cycles are extended significantly, allowing for the potential use of a simple, fixed bed operation, in contrast to the current standard reactor technology employing more complex continuous catalyst regeneration.
In one embodiment, therefore, the present invention is a process for the dehydrocyclodimerization of C2-C6 aliphatic hydrocarbons comprising contacting a feed stream with a catalyst comprising a gallium component, a zeolite support having a silica to alumina molar ratio greater than about 20 and a pore diameter from about 5 xc3x85 to about 6 xc3x85, and an essentially non-acidic binder at dehydrocyclodimerization conditions to yield a hydrogen gas stream and a product stream containing C6-C9 aromatic compounds.
In a more specific embodiment the present invention is a process for the dehydrocyclodimerization of C2-C6 aliphatic hydrocarbons comprising contacting a feed stream with a catalyst comprising a gallium component, an MFI structure type zeolite support having a silica to alumina molar ratio greater than about 20 and a pore diameter from about 5 xc3x85 to about 6 xc3x85, and a silica binder at dehydrocyclodimerization conditions to yield a hydrogen gas stream and a product stream containing C6-C9 aromatic compounds, present in an amount of least about 50% of the weight of the converted C2-C6 aliphatic hydrocarbons.
In another embodiment the present invention is a process for preparing a catalyst for the dehydrocyclodimerization of C2-C6 aliphatic hydrocarbons, the process comprising forming a bound zeolite comprising a zeolite having a silica to alumina molar ratio greater than about 20 and a pore diameter from about 5 xc3x85 to about 6 xc3x85, and an essentially non-acidic binder; calcining the bound zeolite at a temperature from about 450xc2x0 C. to about 700xc2x0 C. for a period from about 1 to about 20 hours to yield a calcined composite; contacting the calcined composite with an aqueous solution of a gallium metal salt selected from the group consisting of gallium nitrate, gallium chloride, gallium bromide, gallium hydroxide, and gallium acetate to yield a gallium-impregnated composite; reducing the gallium-impregnated composite at a temperature from about 400xc2x0 C. to about 700xc2x0 C. for a period from about 1 to about 10 hours in a hydrogen-containing gas to yield a reduced gallium impregnated composite; and oxidizing the reduced gallium impregnated composite at a temperature from about 400xc2x0 C. to about 700xc2x0 C. for a period from about 1 to about 10 hours in an oxidizing atmosphere comprising air and steam where the steam is present in an amount from about 1% to 100% of saturation to yield the catalyst.
In a final embodiment the present invention is a catalyst made by the above-described process.